what Avid IS adapter to I need for my Trek Fuel?

I've got a 2002 Trek Fuel 100 carbon frame that I am refreshing/upgrading. I bought a pair of Avid BB7 brakes to install. But I don't know what IS adapter to get/use. The included adapters are too long (bolt holes spacing)

The rear dropout of the Trek has a set of "IS" holes to mount a disc brake that are about 44mm apart. I.e., closer together than the 51mm standard that the included adapters are.

Does Avid make a "44mm" IS adapter? And, if so, what stand-off height would I need to get the caliper positioned properly for the 160mm rotors? Is there a way to calculate this?

You have the BB7 brakes so you have the fitting instructions which have all (three) available rear adapters listed. As far as I know and remember all the bikes I've had going back to the early 2000s had a 51mm. IS mount. I own a great German made "Steppenwolf" bike on which I'd like to fit a rear disc brake. (Got a BB7 on the front.) It also has two threaded holes on the seat stays near the dropout which are about 4cm. apart. I always assumed they were for mounting a fender or carrier as there are the same holes on both sides. I doubt that you can mount a caliper there but if you find anything let me know.

There is the Brake Therapy option but at the price I'll stick with the V-brake on the back.

The trouble with having an open mind is that people will insist on trying to put things in it.

You will need this bolt on mount and then you'll need a IS to PM adapter of the appropriate size to suit the rotor that you'll be using. Trek started using this bolt on mount a couple of years ago on their 3 Series bikes, your local Trek dealer should be able to order you one of these mounts.

I ended up buying the second one you suggested: the 160mm integrated adapter, from "Sellution Components" on eBay. Hope to receive it soon (the seller only lives 10 miles from me, but 3 days in-transit so far? Love USPS!)

Hopeful that this will result in a clean mount that doesn't flex too much. I'll post my results.
Thanks Cobba, for taking the time to post such a thorough response!